Naming of ready/valid interface

**Type of issue**: feature request

**Impact**: API addition (no impact on existing code), later API modification

I would like to give the ready/valid interface a better name and also a better nome for the data field.

Interface name: It is not clear to me what DecoupledIO means. Who is decoupling from whom? A more logical name for the ready/valid interface would be ReadyValidIO.

Data field: this field is normally (e.g., in Dally) simply named data. Why bits?

I know renaming might be painful, but it has been done. And if it gives in the long run better reading (code understanding) it might be worth it.

Start by locating the DecoupledIO interface and its bits field, then review their usages across the Chisel API. Check the issue discussion before proceeding; done means the proposed names and any compatibility or migration impact are resolved consistently.

Written by the indexing model from the issue text.